There is none. A huge misconception is that a student must major in a science discipline to get into medical school; this is as far from the truth as you can get. Medical schools do not give any preference into what you major in.
Medical schools have required science classes that you must take to apply to medical schools. These courses generally include biology, chemistry, mathematics, and physics. Apart from these courses, you are not required to take any other specific classes. So whether you major in biology, economics or history, you can apply to medical school as long as you meet the requirements.
Our advice is that you major in something that you like and excel at. The reason is because medical schools care more about how you did in the courses you took, then about what courses you took. If you like something and are good at it, you probably will do well in it.
One word of caution though is that medical schools see two different GPAs: science GPA and overall GPA. Overall GPA takes into account all of your courses while science GPA considers only the science classes you have taken. Science GPA matters more than overall GPA though overall GPA is still important. So if you are going to major in a non-science discipline, make sure to do well in all your required science classes because those are the only classes that will make up the science GPA.
To learn more about this question, read You Can Major in Anything.